In ophthalmology, devices known as keratometers have been used to measure the curvature of radius of the cornea. One method is to measure an image size for a fixed object. In another approach, the image size is caused to be fixed by varying the size of the object viewed through a microscope after reflection from a cornea. Systems involving the use of the calibration on zoom lenses have also been employed in the keratometer art.
Difficulty has been encountered in various systems by reason of the fact that the human eye is not quiescent, but constantly moves back and forth, making difficult certain of the measurements involved.